The present invention relates to thickness measuring apparatus, and particularly to a thickness measuring apparatus for measuring the thickness of an object having a transparent or light-transmittable portion such as a semiconductor wafer.
For example, for the measurement of the thickness of a semiconductor wafer as an object having a transparent or light-transmittable layer, it is necessary that the measuring apparatus be able to determine a thickness of about 800.about.100 .mu.m with a precision of about 1 .mu.m before and after grinding the rear side of the semiconductor wafer.
One of the conventional methods of measuring the thickness of such a semiconductor wafer employs two electric micrometers. In this method, the two probes or two measuring contact ends of the two electric micrometers are pressed against the front and rear sides of the wafer, respectively so that the micrometers can determine the positions of the front and rear sides and then find the wafer thickness from the measured front and rear side positions.
In this method, however, since the probes or the contact ends of the electric micrometers are pressed at a constant pressure against the wafer sides, particularly the front side where electric circuits are printed is damaged (injured) by the contact end. Thus the measured object must be discarded.
Another method for determining the thickness of, for example, a semiconductor wafer as an object having a transparent layer utilizes a triangulation type laser displacement gauge. In this method, as exemplarily illustrated in FIG. 1, two laser displacement gauges 14-1, 14-2 of the triangulation type are disposed above and below a semiconductor wafer 10, respectively. The semiconductor wafer 10 is positioned, and measured about its front and rear side positions by the laser displacement gauges. Then, the thickness of the wafer is calculated from those measurements. This triangulation type laser displacement gauge has a PSD (position sensitive device, this abbreviated name will hereinafter be used) as a displacement sensor. This PSD, as illustrated in FIG. 2, is constructed by a high-resistance silicon semiconductor with a resistance layer (for example, P layer) formed as a PN junction on the surface to have a photoelectric effect. When light is incident to the PSD, the light is converted into a photocurrent. The photocurrent flows between the ends of the resistance layer and is taken out from the electrodes. Since the resistance layer has a uniform resistance over the entire surface, the photocurrent is dividedly taken out from the electrodes in inverse proportion to the distance from the light incident position to each electrode. Thus a photocurrent according to the amount of light and the incidence position is detected, and barycentric position of incident light is detected. Such a triangulation type laser displacement gauge is known as UltraGage 9510.TM. etc. manufactured by ADE corporation.
A description will be made of the measurement of, for example, the thickness of the wafer 10 having a transparent film 101 shown in FIG. 1 as an object being measured. A laser light beam I emitted from a laser light oscillator 141-1 of the laser displacement gauge 14-1 located above the wafer 10 is passed through a lens and irradiated on the surface of the wafer 10. A laser beam R1 reflected from the surface of the transparent film 101 and a laser beam R2 reflected from the bottom side 103 of the transparent film 101 are converged at two points 145, 146 on a PSD 143-1, respectively.
The PSD 143-1 does not detect separately the positions of the two points 145, 146, but detects one barycentric position of light depending on the amounts of light at the two points and the positions of the points as the surface position of the wafer 10. Therefore, this measuring apparatus cannot correctly detect the surface position (the position of the surface 102 of the transparent film 101) of the wafer 10. In that case, if the wafer 10 has a transparent film of, for example, about 10 .mu.m thickness, there occurs a serious problem that the thickness of the wafer 10 will be measured with error of about several microns. In FIG. 1, reference numeral 14-2 represents a triangulation type laser displacement gauge for measuring the position of the rear side of the wafer 10, and 143-1 and 143-2 represent the laser beam oscillator and PSD of the gauge.
Another method of measuring the thickness of semiconductor wafer 10 shown in FIG. 1 as a measured object having a transparent layer employs an electrostatic capacitance type displacement sensor in place of the triangulation type laser displacement gauge. In this method, the two electrodes are pressed against the front and rear sides of the wafer 10, respectively to measure the electrostatic capacitance of the wafer, thereby determining the thickness. In this method, however, since the transparent film 101 and the underlying non-transparent layer (reflecting layer) 105 are both insulating layers, the thickness of each layer cannot be measured correctly even though the dielectric constants of these layers 101, 105 are known. Thus, the thickness of wafer 10 including the thickness of the transparent film cannot be measured correctly.